A few of us on the Apache HTTP Server documentation team are
investigating what it would take to add a user commentary system to the
Apache HTTPD docs, like that used on the PHP docs. What software is used
for that? Is it easily reusable in other places? Where can I get some
more info about it?

It's custom code for our docs, but it probably wouldn't be too hard to
bolt onto another system. Start here:

See the various manual_note* functions in that file. Also note that
people who are "logged in" by having the MAGIC_COOKIE set can click
directly on a note to reject it or edit it on the master server from the
manual page instead of having to go hunting for it manually in the note
console.